1. Field of the Invention
The present invention relates to bidirectionally transmittable optical wavelength division multiplexed (WDM) transmission systems.
2. Description of the Related Art
In conventional bidirectional WDM transmission systems, optical signals are optically amplified by the following methods. One method is to perform bidirectional WDM transmission in full duplex using two optical fibers. The other is, as disclosed in, for example, Japanese Patent Publication Laid-Open No. 11-284576, to separate an optical signal traveling through a single optical fiber into a plurality of separated optical signals respectively having different wavelength bands using WDM couplers or circulators, and thereafter to perform optical amplification using discrete amplifiers. Further, Japanese Patent Publication Laid-Open No. 6-222412 discloses, in its latter part, a method of carrying out bidirectional pumping using distributed amplifiers for upstream and downstream optical signals each having the same wavelength band. That is, when distributed amplification is applied to bidirectional WDM transmission using a single fiber, pumping light sources are disposed at both ends of the transmission path, whereby the optical signals are pumped in both forward and backward directions.
The “distributed amplifier” herein means a device that utilizes the characteristics of a transmission path itself and thus amplifies optical signals while transmitting the optical signals therethrough. A distributed Raman amplification is an example. On the other hand, the “discrete amplifier” means a device that amplifies optical signals using an amplifying medium independent of a transmission path. An erbium-doped fiber amplifier (EDFA) is an example of such a medium.
For long-distance WDM transmission systems, it is known that the distributed amplifiers are effective in increasing optical repeater spacing and improving optical SNR, i.e., increasing transmission distance, as discussed, for example, by T. Terahara et al. in “128×10.66 Gbit/s Transmission over 840-km Standard SMF with 140-km Optical Repeater Spacing (30.4-dB loss) Employing Bidirectional Distributed Raman Amplification” (OFC′00, PD28, 2000).
However, to use distributed amplifiers in two-way communications, the following problem must be considered. A distributed amplifier can amplify an optical signal by two techniques: forward pumping shown in FIG. 1 in which pumping light is supplied codirectionally with the optical signal for pumping, and backward pumping shown in FIG. 2 in which pumping light is supplied counter-directionally to the optical signal. Of these two amplifying techniques, forward pumping is known to cause interference between the optical signal and the pumping light since the optical signal and the pumping light travel in the same direction, and this degrades transmission quality, as discussed in the paper prepared by Yasuhiro Aoki, Shunji Kishida, and Kunihiko Washio (“Stable cw backward Raman amplification in optical fibers by stimulated Brillouin scattering suppression”, Applied Optics, Vol. 25, No. 7, p. 1056, 1986).
This paper also indicates that backward pumping gives a solution to the problem of degraded transmission quality.
Further, forward pumping causes the intensity of pumping light to exceed that of an incident optical signal in a transmission-path optical fiber, and would hence be affected largely by nonlinear optical effects, depending on the intensity of the incident optical signal.
Still further, the distributed amplifier could produce gain differences among wavelengths (channels) multiplexed into a single wavelength band.
Even further, in the event that gain provided by the distributed amplifiers is insufficient, the system requires use of discrete amplifiers in addition to the distributed amplifiers for continuous transmission.